Electron gun assembly with long life annular cathode curvilinear electron flow



May 10, 1966 J 3,250,936

; PUA ELECTRON GUN ASSEMBLY WITH LONG LIFE ANNULAR CATHODE O'U'RVILINEARELECTRON FLOW Filed June 28, 1962 INVENTOR JAMES PUA;

HIS ATTORNEY.

AXIALDISTANCE FROM REAR EDGE OF' CATHODE lN MILS United States Patent3,250,936 ELECTRON GUN ASSEMBLY WITHLONG LIFE ANNULAR CATHODECURVILINEAR ELEC- TRON FLOW James Pua, Liverpool, N.Y., assignor toGeneral Electric Company, a corporation of New York Filed June 28, 1962,Ser. No. 205,979

3 Claims. (Cl. 313-85) The present invention relates to a novel electrongun assembly having a cathode of appreciably extended useful life.

The present gun assembly has application to many types of electrondischarge devices wherein the cathode life may be limited for suchcauses as high evaporation rates due to high filament heatingtemperatures, and also because of cathode bombardment which may producenumerous undesirable effects, e.g., poisoning, oxidation and sputteringof the cathode emissive surface.

For example, in conventional display and storage tubes, the generationof a relatively high density, small spot size beam normally requires acathode having a restricted emissive area. To provide the required highemissivity for such operation, it is necessary to employ high filamentheating temperatures which cause rapid evaporation of the cathodeemissive surface. This can be a limiting factor with respect to theoperating life of the cathode. In addition, since the emissive surfaceof these cathodes must be in axial alignment with the generated electronbeam, they are subjected to the previously noted undesirable effects ofcathode bombardment caused by backwardly directed positive ions andcharged particles. With respect to the employment of oxide coatedcathodes, cathode poisoning results in the surface becoming nonemissive.This is due to a change in its chemical composition caused by thecombining of impinging ions with the metal oxide coating. Oxidation ofthe cathode oc- I curs when the emissive surface is oxidized by thebombarding elements, thereby increasing the surface evaporation. Thismay be of significance in limiting the life of both oxide coated andpure metal cathodes. A sputtering or chipping of the cathode surface mayalso be a significant effect in destroying the emissive surface of thesecathodes.

The problem'of cathode bombardment becomes particularly severe in tubeswhere excessive impurities are present, such as in demountable vacuumsystems wherein relatively poor vacuums are obtained. In particular, incertain types of demountable tube structures, such as are employed inrecently developed thermoplastic tape recording systems, the effects ofcathode bombardment are great. For example, in such systems theoutgassing from the recording medium contained within the vacuum systemproduces heavy organic charged particles and water vapor whichcontribute greatly to the impurities present.

In addition, in high power tubes such as many travelling wave tubes,wherein axial alignment of the cathode emissive surface is normallyrequired, the operating life of the cathode, in many instances, isseriously effected by cathode bombardment. A The gun assembly of thepresent invention is constructed so as to be free of the above notedadverse effects of cathode bombardment, has reduced filament heatingtemperature requirements, and further is capable of providing aconcentrated high density electron beam where required.

Accordingly, it is an object of the present invention to provide animproved and readily constructed electron gun assembly which employs acathode of extended useful life.

It is another object of the present invention to provide an improvedelectron gun assembly of relatively simple construction for generating aconcentrated electron beam from a cathode exhibiting an extended usefullife.

It is another object of the present invention to provide an improvedelectron gun assembly of relatively simple construction for generatingan electron beam of very high density.

It is still another object of the present invention to provide animproved electron gun assembly of relatively simple construction havinga cathode of reduced current density loading and one relatively free ofdeleterious effects of cathode bombardment.

It is a further object of the present invention to provide an improvedelectron gun assembly which generates an electron beam for use in anelectron discharge device having impurities present, said gun assemblybeing of relatively simple construction and employing a cathodeunaffected by said impurities and having an appreciably extended usefullife.

It is still a further object of the present invention -to provide animproved electron gun assembly which generates a concentrated electronbeam for use in a de' mountable vacuum system, said gun assembly beingof relatively simple construction and employing a cathode of reducedcurrent density loading and one that is relatively free from thedeleterious effects of cathode bombardment.

In accordance with one aspect of the invention, there is provided anelectron gun assembly for use in an electron discharge device having aring type cathode in combination with first and second electrodes, eachhaving central apertures, in axial alignment with the cathode andconsecutively disposed in the order recited immediately forward of saidcathode, and a third electrode in axial alignment with the cathode anddisposed immediately rearward thereof. Said second electrode has anaxially protruding central portion of cylindrical configurationextending within the aperture of said first electrode. The cathode,first, second and third electrodes are energized so as to provide in thevicinity of said cathode and said first and second electrodes anelectric field configuration which accelerates the emitted electronsalong a curvilinear path through the apertures of said first and secondelectrodes into a restricted crossover region which forms the apex of aconcentrated electron beam.- Accordingly, a composite electric field isestablished in the initial region of electron travel having an axialcomponent in the forward direction provided essentially between thesecond and third electrodes and a radial component extending towards theaxis of alignment provided essentially between the first electrode andthe protruding portion of said second electrode. The described electrodeconfiguration and electric field produces a concentrated electron beamhaving as its source a relatively large emissive area of annularconfiguration. In addition, the backwardly directed positive ions willbe accelerated in the axial direction so as to strike said thirdelectrode and not thecathode.

While the specification concludes with claims particularly pointing outand distinctly claiming the subv exemplary embodiment of applicantselectron gun as- 3,25ltl,936

3 sembly mounted within a demountable discharge device, which isenclosed by a glass envelope 2, of the type in which a concentrated beamis generated, only the gun portion of the device being shown. The gunassembly 1 employs a ring type cathode 3 having an electron emissiveinner surface, a cup shaped back electrode 4 positioned to the rear ofthe cathode 3, and positioned immediately forward of the cathode inconsecutive manner, a cylindrically shaped focusing electrode 5 havingplane end surfaces, a first disk shaped anode electrode 6 and a secondanode electrode 7, each electrode, except back electrode 4 having acentral aperture. The cathode 3 and electrodes 4 to 7 are aligned alonga longitudinal axis of the discharge device. A shield 8 is fitted aroundcathode 3 and electrodes 4 and 5 to exclude extraneous field effects inthis region The first anode 6 has an axially protruding portion 9 ofgenerally cylindrical configuration which extends within the aperture offocusing electrode 5. The construction and relative dimensions of thecathode 3 and electrodes 4, 5 and 6 are shown more clearly in FIGURE 2.

Mounting rings 10 and 11, conductive rods 12 and 13,

and insulator members 14 and 15 are employed for supporting the cathode3 and electrode structures 4 to 7 in a conventional manner. Rods 12 and13 also provide a current path for heating of the cathode 3. Conductors16, 17, 18 and 19 apply operating potentials to electrodes 4, 5, 6 and7, respectively. Base terminal pins 20 form a part of the supportingstructure and also introduce the operating potentials into the dischargedevice through a vitreous base member 21 A vacuum seal, such as aneoprene O-ring 22, is inserted between base 21 and glass envelope 2.

The cathode ring 3 may be constructed from a number I of suitablematerials. For example, it may be constructed of a pure molybdenum ortungsten metal, or it may be an oxide coated metal with the coatingplaced on the inner surface of the ring. In one operating embodimentmolybdenum was employed. In the width of the emissive surface of thecathode was approximately 16 mils and the diameter of its aperture about150 mils, providing an emissive area about 10 times that of a hairpintype cathode of comparable total emissivity. The cathode was heated toabout 2150 K., a considerably lower temperature than required forrestricted emissive area cathodes. Hence, the rate of evaporation of theemissive surface is appreciably reduced. There was obtained a prefocusedbeam current density in the crossover region of approximately .83amperes per square inch from a cathode current density of approximately.0037 amperes per square inch. The lower heating temperaturerequirements, in addition to the avoidance of bombardment of the cathodewhich will be explained in more detail subsequently, results inextending the useful operating period of the cathode by more than anorder of magnitude over conventional structures providing comparablebeam intensity. It may be noted that the annular emissive surfaceconfiguration as a characteristic of its large area emissive surfaceprovides a further advantage in that axial alignment of the cathode neednot be as critical at it is required to be for restricted emissive areacathodes. In addition it may be appreciated that the large cathodeemissive area provided will permit the generation of a very high densitybeam current when the cathode is heated close to the maximum allowableheating temperature.

The diameter of the cavity of back electrode 4 and the diameter of theaperture of focusing electrode 5 are approximately equal to the diameterof the cathode aperture. The diameter of the aperture of theaccelerating electrode 6 is approximately of the first reciteddiameters. In the operating embodiment being considered, the thicknessof the focusing electrode 5 was about mils, which dimension provides anopposing surface with respect to the protruding portion 9 of the anode 6and will be seen to be significant with respect to generation of theradial component of the electric field in the region of initial electrontravel. The second accelerating anode 7 has a limiting aperture thediameter of which was on the order of 4 mils. It should be understood,however, that, for purposes of the invention, the second anode isprimarily for the purpose of providing a strong axiallydirected electricfield for causing further convergence of the beam. It thus need notinclude a limiting aperture and may be of conventional constructionother than that illustrated, and it may actually be a part of the lensof the subsequent focusing system, which itself is of conventionalconstruction and is not shown.

The cathode 3, back electrode 4 and focusing electrode 5 are biased atapproximately the same potential, normally at ground potential.Accordingly, these structures can be considered as a single electrodemeans the inner surface of which forms a cylinder closed at one end. Aninput signal is normally applied between the focusing electrode 5 andthe cathode 3 for modulating the electron beam. In the embodiment of theinvention considered, a potential of approximately volts was applied tothe first accelerating anode electrode 6 and a potential ofapproximately 450 volts was applied to the second accelerating anodeelectrode 7. The various bias potentials may be supplied from a tappedpotentiometer 23.

An electric field is established in the vicinity of cathode 3 whichdraws electrons away from the inner surface thereof in a curvilinearpath which extends through the apertures of electrodes 5, 6 and 7, asillustrated in FIG URE 2. It may be noted that when considering puremetal cathodes, it is essentially the inner surface only that isemissive because a negligible accelerating field exists at the othersurfaces. Thus, the emitted electrons converge in .a constrictedcrossover region in the vicinity of the limiting aperture of electrode 7so as to form a high density beam which may be focused onto a phosphorscreen or recording surface by the focusing system, not illustrated. Theequipotential lines of the electric field in the region of the initialelectron travel are shown in FIGURE 3. This electric field configurationis essentially provided by an axial field component existing between thefirst anode electrode 6 and the back electrode 4, along the longitudinalaxis of the discharge device towards said first anode 6, and a radialfield component existing between the protruding central portion 9 ofanode 6 and the focusing electrode 5, in the direction towards thelongitudinal axis. The axially extending side portions of the cup-:shaped back electrode 4 serve to confine the electric field existing tothe rear of the cathode and contribute to providing an optimizedelectric field configuration in the -vicinity of the cathode. Further,the back electrode 4 serves to establish an axial field component whichwill provide a smooth curvilinear path of electron flow converging in aregion forward of the anode electrode 6. Accordingly, the disclosedelectrode configuration and applied potentials provide an electric fieldin the region between the cathode emissive surface and the aperture ofanode electrode 6 having a radial and axial component, said radialcomponent being a maximum in the vicinity of the emissive surface,gradually decreasing and "becoming minimum in the vicinity of the anodeaperture and said axial component being a minimum at the emissivesurface, gradually increasingand becoming maximum in the vicinity of theanode aperture. The described electric field accelerates emittedelectrons away from the cathode in a gradually curved path bending inthe forward axial direction of the discharge device, as seen in FIGURE3. In addition, the equipotential lines of the electric field are of aconfiguration such that any positive ions or charged particles which arebackwardly accelerated through the apertures of the gun electrodes willnot strike the emissive surface of the cathode but will proceed in adirection essentially along the longitudinal axis and be collected bythe back electrode 4.

It may be appreciated that although the present electron gun assemblyhas been described as useful with electron discharge devices whichgenerate high density solid beams, it may be of appreciable value forthe generation of hollow beams. In addition the invention hasapplication for employment in travelling wave type tubes, which tubesnormally utilize a spherical cathode positioned on the longitudinal axisof the device. Such cathodes are often subjected to intense bombardmentby backwardly directed positive ions which distort the emitted beamconfiguration and ultimately cause cathode failure. Applicants inventionwould prevent such phenomenon and accordingly lengthen the life of thecathode in tubes of this type.

Accordingly, the appended claims are intended to be construed asembodying all modifications and variations that fall within the truescope and spirit of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An electron gun assembly for generating an electron beam in anelectron discharge device comprising:

(a) a cathode having an electron emissive surface of annularconfiguration,

('b) first and second annular electrodes each having an aperturetherein, said electrodes being arranged in consecutive manner forward ofsaid cathode and in coaxial relationship therewith, said secondelectrode having an axially protruding central portion extending withinthe aperture of said first electrode, and

(c) a third electrode having a cavity therein disposed immediatelyrearward of said cathode, said first, second and third electrodes beingenergized so as to create an electric field having a predominantlyradial component in the vicinity of said cathode and a predominantlyaxial component in the vicinity of the aperture of said second electrodewhich accelerates electrons from said emissive surface in a convergingcurvilinear path passing through the aperture of said second electrode.

2. An electron gun assembly for generating an electron beam in anelectron discharge device comprising:

(a) a ring shaped cathode having an electron emissive inner surface,

(b) .a cylindrical focusing electrode positioned forward of said cathodeand having an aperture therein the diameter of which is approximatelyequal to the diameter of said cathode,

(c) an anode electrode positioned forward of said focusing electrode andhaving an axially protruding central apertured portion extending withinthe aperture of said focusing electrode,

(d) a cup-shaped back electrode positioned immediately rearward of saidcathode, and

(e) means for applying substantially equal bias potentials to saidcathode, focusing and back electrodes and a relatively positivepotential to said anode electrode for providing an electric field havinga predominantly radial component in the vicinity of said cathode and apredominantly axial component in the vicinity of said anode aperturewhich accelerates electrons from said emissive surface in a convergingcurvilinear path passing through said anode aperture.

3. Anelectron gun assembly for generating an electron beam in anelectron discharge device comprising:

(a) a cathode having an electron emissive surface of annularconfiguration,

(b) electrode means including a first apertured, cylindrically shapedelectrode having plane end surfaces positioned immediately forward ofsaid cathode, and a second apertured, disk shaped electrode positionedcoaxially with and immediately forward of said first electrode andhaving an axially protruding central portion extending within theaperture of said first electrode, and

(c) a third electrode disposed immediately rearward of said cathode inapproximate axial alignment with the apertures of said first and secondelectrodes, said first, second and third electrodes being energized soas to create an electric field having an axial and radial component themagnitudes of which are inversely related and gradually varying, saidradial component being maximum in the vicinity of said emissive surfaceand said axial component being maximum in the vicinity of the apertureof said second elect-rode, said electric field thereby acceleratingelectrons from said emissive surface in a converging curvilinear pathpassing through the aperture of said second electrode.

References Cited by the Examiner UNITED STATES PATENTS 2,547,200 4/1951Dorgelo 313-64 2,813,990 11/1957 Robertson 313 2,935,642 5/1960 Schwartz3l515 OTHER REFERENCES 1958 IRE National Convention Record, vol. 6, part3,

pages 13-20, article The Annular Geometry Electron Gun, by James W.Schwartz.

DAVID J. GALVIN, Primary Examiner.

V. LAFRANCHI, Assistant Examiner.

3. AN ELECTRON GUN ASSEMBLY FOR GENERATING AN ELECTRON BEAM IN ANELECTRON DISCHARGE DEVICE COMPRISING: (A) A CATHODE HAVING AN ELECTRONEMISSIVE SURFACE OF ANNULAR CONFIGURATION, (B) ELECTRODE MEANS INCLUDINGA FIRST APERTURED, CYLINDRICALLY SHAPED ELECTRODE HAVING PLANE ENDSURGACES POSITIONED IMMEDIATELY FORWARD OF SAID CATHODE, AND A SECONDAPERTURED, DISK SHAPED ELECTRODE POSITIONED COAXIALLY WITH ANDIMMEDIATELY FORWARD OF SAID FIRST ELECTRODE AND HAVING AN AXIALLYPROTRUDING CENTRAL PORTION EXTENDING WITHIN THE APERTURE OF SAID FIRSTELECTRODE, AND (C) A THIRD ELECTRODE DISPOSED IMMEDIATELY REARWARD OFSAID CATHODE IN APPROXIMATE AXIAL ALIGNEMENT WITH THE APERTURES OF SAIDFIRST AND SECOND ELECTRODES, SAID FIRST, SECOND AND THIRD ELECTRODESBEING ENERGIZED SO AS TO CREATE AN ELECTRIC FIELD HAVING AN AXIAL ANDRADIAL COMPONENT THE MAGNITUDES OF WHICH ARE INVERSELY RELATED ANDGRADUALLY VARYING, SAID RADIAL COMPONENT BEING MAXIMUM IN THE VICINITYOF SAID EMISSIVE SURFACE AND SAID AXIAL COMPONENT BEING MAXIMUM IN THEVICINITY OF THE APERTURE OF SAID SECOND ELECTRODE, SAID ELECTRIC FIELDTHEREBY ACCELERATING ELECTRONS FROM SAID EMISSIVE SURFACE IN ACONVERGING CURVILINEAR PATH PASSING THROUGH THE APERTURE OF SAID SECONDELECTRON.